Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity.
Applicants have recognized that under some operating conditions, steps taken to mitigate a cylinder misfire can also lead to an increased likelihood of pre-ignition. Specifically, in response to a cylinder misfire event, an engine controller may shut-off fuel to the misfiring cylinder to prevent an exhaust catalyst from over-heating. Additionally, the remaining cylinders may be operated leaner than stoichiometry to reduce the amount of unburned fuel. However, the lean cylinder operation may increase the propensity of engine pre-ignition, particularly at higher engine speeds, and expedite engine degradation.
The above issue may be at least partly addressed by a method of controlling a vehicle comprising, while driving the vehicle with an engine, operating a cylinder to pump air without injected fuel while another cylinder combusts a lean air-fuel mixture, and limiting air into the cylinders to be less than a threshold. In this way, by limiting the engine load when some cylinders have fuel shut off and other cylinders are operating lean, the propensity for inducing engine pre-ignition can be reduced.
In one example, in response to a misfire event in a first engine cylinder, fuel injection to the misfiring cylinder may be shut off while air continues to be pumped there-though. The remaining cylinders may then be operated with an air-to-fuel ratio that is leaner than stoichiometry to reduce the amount of unburned fuel remaining in the cylinders. To then reduce the likelihood of cylinder pre-ignition that may be induced by the lean operating conditions, in particular at medium to high engine speeds, an engine load may be limited. The load limiting may be based on the leanness of the lean air-to-fuel ratio as well as the number of cylinders running lean (or a number of cylinders operating with fuel shut-off). The load limiting may also be based on the engine's pre-ignition history (indicative of the engine's inherent propensity for pre-ignition), as well as the engine speed. As such, the amount and duration of load limiting may be adjusted to enable exhaust temperatures to be controlled and the likelihood of pre-ignition to be reduced. After the defined duration has elapsed, the load limiting may be ramped out.
In this way, by limiting an engine load during conditions when some cylinders are running lean and other cylinders have fuel shut-off, over-heating of an exhaust catalyst and related component degradation may be reduced. By reducing engine temperatures, the propensity for a cylinder pre-ignition event can also be reduced. By limiting an engine load during misfire mitigation, the likelihood of a pre-ignition event being induced by the misfire mitigation can also be reduced. Overall, engine degradation can be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.